1. Field of the Invention
The invention relates to an X-ray device, including an X-ray source for emitting an X-ray beam, an X-ray image converter for detecting X-rays and for forming an X-ray image, a primary diaphragm device which, in order to limit the X-ray beam, is arranged between a radiation source and an object to be examined, and a control circuit for automatic exposure control, including a detector with at least one measuring field measuring the X-ray dose rate.
2. Description of Related Art
An X-ray device of this kind is known from DE-A 30 06 774. The control circuit thereof includes an adder which superposes two memory values on a reference value, the first memory value incorporating the dependency of the conversion factor of an X-ray image intensifier, used as the X-ray image converter, on the electrode voltage whereas the second memory value incorporates the dependency of the background (the residual brightness) of the X-ray image intensifier on the aperture of the primary diaphragm device which determines the image format. Measurement values for the automatic exposure control, falsified by the X-ray image intensifier, are thus corrected.
The area of diagnostic interest in the X-ray image should have a given mean density. The measuring field of the detector is made to register with this area of interest, and during the exposure the already applied X-ray dose is continuously measured in this measuring field by measurement of the X-ray dose rate. When a predetermined X-ray dose is reached, corresponding to a mean density, the automatic exposure control switches off the X-rays.
In order to adapt the dimensions of the area traversed by the X-ray beam to the medical requirements and to reduce, if necessary, the contrast-degrading effects of scattered radiation, a primary diaphragm device for limiting the X-ray beam is arranged between the X-ray source and an object to be examined, for example a patient. However, if limiting is so extensive, that is to say if the diaphragm aperture is made so small, that parts of the measuring field are masked by the shutters of the primary diaphragm device so that these parts are no longer exposed, overexposed X-ray images will be obtained when the conventional exposure control system is employed. This is because the detector measures the dose in the entire measuring field by averaging, even when areas of the measuring field are masked by the shutters so that they are not exposed. Therefore, the dose averaged across the entire measuring field in the case of extensive limiting is less than the dose obtained in the exposed area of the measuring field. The conventional automatic exposure control then increases the X-ray dose until the predetermined X-ray dose is reached. This often results in overexposed X-ray images, in which important image information is lost, and also leads to an increased dose for the patient.